1. Field of the Invention
The present invention relates to a method and apparatus for reading a bar code of a photographic film cassette of a camera, and more particularly to a method and apparatus for reading a bar code written on a bar code disc of a photographic film cassette while the film is advanced.
2. Description of the Related Art
There is known a photographic film cassette or cartridge of the type that the leader of the photographic film is advanced out of the cassette while the cassette spool rotates. U.S. Pat. No. 5,049,912 describes an improved version of the photographic film cassette of this type. This photographic film cassette has a bar code disc mounted on the cassette spool, the bar code disc being provided with written information specific to the film cassette, such as the sensitivity of the loaded film and the number of frames capable of being photographed.
The bar codes include black bars and white bars (spaces). Each bar includes a wide bar and a narrow bar in order to represent binary information. These bars are measured by a reflection type bar code sensor, and in accordance with measured signal widths, the widths of bars representing logical values "0" and "1" are determined. It is necessary for the measurement of a bar width to rotate a bar code disc at a constant angular velocity in the camera. If a rotation speed of a cassette spool is unstable, it becomes necessary to measure each bar by a bar code sensor, by using a high precision rotary encoder coupled to the bar code disc and generating pulses at each fine unit angle and by checking a feed amount of the photographic film.
The rotation speed of a motor for driving a camera spool in a film take-up chamber and for transporting a photographic film in the camera changes with an environmental temperature and camera battery condition. Even if the motor rotation speed is constant, the film transport speed changes with the diameter of a film roll wound about the camera spool. As the film transport speed changes, the angular velocity of the cassette spool changes correspondingly so that it is difficult to rotate the bar code disc at a constant angular velocity. If a high precision rotary encoder is used for reading bar codes, it is disadvantageous from the viewpoint of space and cost.
The center of an amplitude of a photoelectrical signal detected by a bar code sensor becomes lower or higher than a threshold value used as a criterion of a signal change, depending upon the remaining capacity of a battery, the position of a bar code disc, a variation of bar code sensor performance, or the like. In such a case, the amplitude (level) of a photoelectrical signal relative to the threshold level increases or decreases so that converted binary signals of "0" and "1" may indicate false values and generate errors of measured bar widths.
U.S. Ser. No. 08/100,322 filed on Aug. 2, 1993 describes a bar code disc recorded with black bar codes having different densities so as to represent logical values of "1" and "0". Although a read error to be caused by a change in an angular velocity of the bar code disc is not present, it is necessary for a correct read of different densities to use a high precision bar code sensor and a binarizing circuit.